de Graaf \də-ˈgräf\ , Reinier (1641–1673), Dutch physician and anatomist. De Graaf is regarded as one of the creators of experimental physiology. In 1672 he produced a treatise on the female reproductive organs that is considered one of the seminal works of biology. De Graaf demonstrated ovulation anatomically, pathologically, and experimentally. The treatise contains the description of the structures now known in English as the graafian follicles. The Swiss anatomist Albrecht von Haller (1708–1777) named them after de Graaf. In 1827 the German embryologist Karl Ernst von Baer (1792–1876) discovered that although the graafian follicles contain the eggs from which organisms develop, they are not the eggs themselves.